CLSM analysis of development and structure of the central nervous system of Enchytraeus crypticus (”Oligochaeta”, Enchytraeidae)

 The phylogenetic position of the Clitellata within the Annelida is still in debate. It is unresolved whether they are a basal group, resembling the stem species of the Annelida, or a highly derived subtaxon, which evolved from a polychaete-like ancestor. A factor often ignored in this discussion is the site of the supraoesophageal ganglion (brain). Its position in the Clitellata, far behind the prostomium, can be regarded as a secondary structural adaptation to a life spent burrowing through relatively firm terrestrial substrates. Such a habit probably led to the complete elimination of prostomial appendages and a marked reduction in size of the prostomium, resulting in a displacement of the brain into following segments. This interpretation of the sequence of events corroborates the view that the Clitellata are a highly derived subtaxon of the primarily marine Annelida. The development of the central nervous system of Enchytraeus crypticus (”Oligochaeta”) was analysed by immunohistochemical methods in combination with confocal laser scanning microscopy. A detailed description of neural structures, including the formation and arrangement of segmental nerves, is given for significant developmental stages. Labelling of neural structures using antibodies against acetylated α-tubulin and serotonin clearly shows that the cerebral ganglion initially develops within the prostomium. In the course of embryonic development it is shifted backwards into segment III. Thus the posterior relocation of the brain, which is inferred as having occurred in the phylogeny of the Clitel- lata, can be demonstrated in the ontogeny of E. crypticus.